ABSTRACT
Like all Christian houses of worship, Saint-Jean Cathedral built between the end of the twelfth and the beginning of the fifteenth centuries was designed with the idea that daylight is radiated by God. As typical in medieval religious architecture, large stained art glass windows, strongly contrasting with adjacent stone walls, admit and colour the light and convey to worshippers the Message of God through translucent illustrations and inscriptions. However, what is particularly interesting in Saint-Jean Cathedral is the evolution of daylight treatment during the long period of construction. The apse was built-in the Roman style with massive walls and focused daylighting, whereupon the nave was built in the Gothic style with a tendency to pierce walls as much as possible. The apse embrasures were built with a stone called 'choin'. The mullions and traceries separating and supporting the former windows were hewn directly from the facade stone walls.
